This invention relates generally to radiation detector array characterization methods and apparatus and, in particular, to methods and apparatus for determining a modulation transfer function (MTF) of a detector array.
FIG. 2a depicts an example of a Line Spread Function (LSF) for a single square detector element 1a, as shown in FIG. 2c. The LSF is a plot of signal intensity output by the detector element 1a in response to a line source 2 that is translated across the detector element 1a. For illustrative purposes, the line source 2 is shown as being translated along an x-axis. From the LSF a MTF for the detector element 1a is determined, the MTF being depicted graphically in FIG. 2b. 
The MTF is a qualitative measure of image quality. Ideally, the MTF is unity for a normalized spatial frequency of zero, and monotonically falls to 0.5 for a maximum spatial frequency of interest. The spatial frequency is a function of the detector pitch.
In an electrooptical system application MTF is typically a quantitative measure of image assessment. Image assessment is a function of a number of factors, including optical quality, diffraction effects, vibrational effects, and, for a finite sized detector element, a blurring induced by the size and non-uniformity across the detector element.
The MTF is a normalized Fourier Transform of the LSF. The normalization results in unity MTF at a spatial frequency of zero. The Nyquist spatial frequency for a staring-type detector array is the reciprocal of twice the detector pitch, where the detector pitch is the center to center spacing between two adjacent detector elements (1a and 1b). The detector elements 1a and 1b are two detector elements of an array of detector elements, such as a staring-type focal plane array (FPA). A typical MTF is a decreasing function as spatial frequency increases.
As employed herein, the Nyquist Frequency, for data defined at equal time intervals (or equal spatial intervals) t, is the frequency of a sine or cosine term with a period double the interval t. Frequencies greater than this amount are not uniquely detectable by spectral analysis.
As an example, for a detector array having a detector pitch of 0.061 mm and a detector width of 0.058 mm (a high fill factor), the Nyquist frequency for the detector array can be expressed as one divided by two times the detector pitch. For the instant example, this yields a spatial frequency of approximately 8.2 cycles/mm. Referring to FIG. 2b, the normalized MTF starts at unity for a spatial frequency of zero, and decreases to 0.5 at the maximum spatial frequency of interest of 8.2 cycles/mm.
Expressed differently, the MTF, for a given sinusoidal spatial frequency, is equal to a maximum detector signal minus a minimum detector signal, divided by the maximum detector signal plus the minimum detector signal.
The MTF is a commonly specified parameter for an optical system. For an optical system that includes relay or imaging optics and a two-dimensional detector array, the overall system MTF is a product of the MTF of the relay optics and the MTF of the detector array.
As such, it is important to accurately determine the MTF of a detector array so as to determine if the MTF of the system meets the specification.
Various techniques are known for determining the MTF of a two-dimensional detector array. These include the following approaches.
In a first approach analog signals output by the detector array are displayed on an oscilloscope screen. A square wave response (SWR) is obtained from:
SWR=(max signalxe2x88x92min signal)/(max signal+min signal).
It should be noted that the MTF is applied to a sinusoidal response, whereas the SWR is a composite of a fundamental sinusoidal component plus higher harmonic components.
A second approach employs a histogram technique. A third approach produces sine wave MTF values. This is a computationally intensive method of constructing a fundamental and its harmonics from a data stream generated by the detector element in response to an illuminated square bar reticle pattern. This method uses an iterative, search optimization methodology. A fourth approach scans a phased knife edge across the detector element. The digitized data stream has one sample per dwell time. The data stream is reconstituted to give a knife edge response (KER). The KER is differentiated to produce a LSF. The normalized Fourier Transform of the LSF gives the sine wave MTF. A fifth approach uses a scanned phased slit source. The data stream is used to obtain a reconstituted LSF which is Fourier Transformed to obtain a sine wave MTF. Other standard test methods are employed when there are no limitations on sampling interval. For example, a single knife edge (or line source 2 as in FIG. 2c) is scanned across a detector producing a well populated KER (LSF). For this case the KER (LSF) is given directly by the data stream output by the detector.
Each of these conventional approaches suffers from one or more of the following disadvantages: a requirement for an accurate optical alignment of a phased slit reticle with the detector array; a requirement that relative motion be provided between the reticle and the detector array; an excessively long computation time; and a MTF characterization of but a single detector element, as opposed to a characterization of the detector array.
The invention disclosed in the above-referenced commonly assigned patent application Ser. No. 07/871,882 overcomes the problems of the prior art by providing method and apparatus for determining the MTF of a detector array, without requiring relative motion between the detector array and a reticle. The use of a phased slit reticle is disclosed, wherein the reticle has a pattern selected to enable the determination of LSF data from the radiation detectors of a row or a column of an array of radiation detectors. The MTF is then subsequently determined from the LSF data.
Although this technique provides superior results, for some applications it is desirable to measure the MTF in a more localized area of the array than that represented by a row or a column.
It is an object of this invention to provide method and apparatus for determining the MTF of one or more localized regions of a detector array, without requiring relative motion between the detector array and a reticle.
It is a further object of this invention to provide a phased slit reticle having a pattern selected to enable the determination of LSF data for a localized region of a radiation detector array, and to thus enable the determination of the MTF from the LSF data obtained from the localized region.
The foregoing and other problems are overcome and the objects of the invention are realized by a method and apparatus for determining the MTF of a detector array that employs a reticle pattern having a plurality of slits arranged in a predetermined pattern for providing a two dimensional phase difference between adjacent slits, the phase difference being a function of detector pitch. Relative motion between the reticle and a detector array under test is not required and, in addition, the alignment requirements are modest.
More specifically, the invention provides both apparatus and method for determining a modulation transfer function of a plurality of radiation detectors. The method includes a step of simultaneously illuminating, with a slit illumination source embodied within a phased slit reticle, a plurality of detector elements that are disposed along rows and columns of a radiation detector array. The phased slit reticle has a two dimensional phase characteristic that is a function of a distance between adjacent detector elements. The method includes a further step of determining, from an electrical signal generated by illuminated detector elements, a line spread function; and a step of determining from the line spread function, the modulation transfer function of the detector elements within a localized region of the radiation detector array.
The phased slit reticle has a first set of parallel slits that are equally spaced apart from one another by a distance (D1), and a second set of parallel slits that are equally spaced apart from one another by a distance (D2). The first set of slits and the second set of slits are orthogonally disposed one to another and define a two dimensional array of cells. The cells of adjacent columns of the two dimensional array of cells are staggered relative to another by an amount equal to a distance (D3). Furthermore, the cells of adjacent rows of the two dimensional array of cells are overlapped with one another by an amount equal to a distance (D4). This staggering and overlap of the reticle cells enables the LSF to be determined within localized areas or regions of pixels of two dimensional array of radiation detectors.
In accordance with a method of the invention a first step irradiates the detector array so as to obtain first reference signals. A next step provides the phased slit reticle PSR having a pattern characterized by a plurality of slits providing a two dimensional phase difference between adjacent slits, the specific phase difference being a function of detector pitch along both rows and columns of the detector array.
A next step irradiates the detector array through the PSR to obtain further signals. These further signals are normalized with respect to the first signals, and the method then determines those detector elements within rows and columns that have an orientation, with respect to the PSR, that is suitable for determining the LSF. Based on this determination detector elements (pixels) are selected from the determined rows and columns. The LSF is generated from a plurality of the selected detector elements that are simultaneously illuminated by the PSR. A further step takes a Fourier transform of the LSF to obtain the MTF performance.